SDRAM (Synchronous Dynamic Random Access Memory) is widely used as a general purpose memory for various products including personal computers. Recently, because of a variety of applications, new general purpose memories, in which the functions and performances have been modified for special usages, have been developed. A low current SDRAM is an example of such a product, and is used for a low-current battery operated product, such as a cellular phone or a PDA (Personal Digital Assistance). Furthermore, an SDRAM having a larger capacity (128 Mb or 256 Mb) than a conventional PSRAM (Pseudo Static Random Access Memory) has started to be used.
In this field, low standby and active currents are required at a slow clock speed such as 100 MHz, which is also lower than that for a general purpose SDRAM. Performance is also low, for example, the access time is 56 ns and the CAS latency is three clocks (26 ns).
Furthermore, the low current SDRAM exploits the same four-bank scheme as the general-purpose SDRAM. The four-bank scheme activates many sense amplifiers at a time and operates many arrays, so that the performance of the low current SDRAM once dropped by a slow clock can be enhanced by multiple accesses through bank activation. Generally, since each I/O activates 512 to 1024 sense amplifiers, a chip with 16 I/Os needs to fire 8K to 16K sense amplifiers in a single access. Thus, it is difficult to reduce the active current that is significantly larger than that of PSRAM.
FIG. 11 shows the typical layout of a low-current 256-Mb SDRAM 1. The SDRAM 1 has four banks, BNK0 to BNK3, and 16 I/Os. The individual bank consists of 32 arrays (ARs), each of which has 1K word lines, 2K bit line pairs and 2K sense amplifiers. Therefore, the individual array has 2M bit cells. To make the 1K-word page size accessible by column addresses, each bank access must fire 16 sense amplifiers. Thus, one row access needs to activate eight arrays, ARs.
As described above, the conventional SDRAM 1 activates many sense amplifiers at a time. Firing sense amplifiers charges up capacitors of the same number of bit line pairs, and the major part of active currents stems from this charging current.